The present invention relates to sealed, prismatic electrochemical cells.
Cylindrical electrochemical cells have a cylindrical housing. Some cylindrical cells contain a roll of thin, flexible electrodes wound up together with a separator layer in between them. This cell construction is sometimes called a "jelly roll", due to the wound configuration of the electrode and separator components. The electrodes of such cells can be made by impregnating porous, sintered metallic plaque substrates with active material, or by applying a paste containing active material onto a metallic substrate. Some other cylindrical cells contain pressed active material powder pellet electrodes arranged in concentric cylinders within the housing, with a separator tube between the opposing electrodes. Cylindrical cells can be relatively inexpensive to make, and the cylindrical shape of the can resists stress concentrations and deformation from internal pressure changes. Standard size AA and A nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries are examples of wound-type cylindrical cells. Standard AA, C and D alkaline batteries are examples of bobbin-type (pellet) cylindrical cells.
Prismatic cells, cells with housings with polygonal side walls (such as parallelepiped or rectangular housings), are found in many applications requiring high power densities, as their shape can provide high volumetric stacking efficiency in battery packs, such as for cellular phones, for instance. A typical F6 nickel metal hydride prismatic cell has three nickel hydroxide positive electrode plaques sandwiched between four metal hydride alloy negative electrode plaques, with separator bags isolating each plaque layer from the next. This electrode stack is inserted into a rectangular metal can, with all of the negative electrode plaques connected to one terminal through a series of metallic tab strips, and all of the positive electrode plaques connected through a series of tab strips to the other terminal. Each of the metallic tab strips is sufficiently insulated to prevent internal shorting between electrode components. Generally, the can itself is one of the two terminals. Prismatic cells are generally more complex and expensive than comparable cylindrical cells, due to the larger number of internal components and attendant assembly operations.
Two important performance characteristics for a battery are its overall capacity (expressed in amp-hours) and its discharge efficiency at a given discharge rate. The rated capacity is a measure of the total amount of usable energy stored in the cell, and relates to the number of hours the cell can power a given load. Capacity is primarily a function of the amount of reactable active material contained within the cell, particularly the amount of whichever active material is first consumed. Typically, cell capacity is measured at a C/5 discharge rate, as described in ANSI C18.2M-1991, published by the American National Standards Institute. The theoretical volumetric capacity of a single electrode is the total energy density of the active material contained within a given volume of the electrode, and can be expressed in ampere-hours per liter. Discharge rate efficiency is affected by the amount of interfacial surface area between the electrodes, and the subsequent degree of polarization which tends to reduce output voltage as the discharge rate increases. The more the interfacial surface area, the higher the discharge rate maintainable above a given voltage, as the discharge rate can be seen as a maximum current per unit of interfacial surface area (current density). A standard nickel metal hydride F6 cell, for instance, may have a total of 32 or more square centimeters of interfacial area between the stacked electrodes.
Polarization, which generally refers to the difference in the open circuit and closed circuit load voltage of the cell, is a function of the current density and consists of three separate terms: activation polarization, ohmic polarization, and concentration polarization. Activation polarization reduces the load voltage at a given load, and is an inherent function of the properties of the active materials chosen for the cell. Ohmic polarization also reduces the load voltage at a given rate due to the collective resistance contributions of the individual cell components, connections and interfaces, and can be reduced by lowering the resistivity of the individual cell components and interfaces. Concentration polarization reduces the load voltage due to diffusion rate limitations of charged ions in and out of the electrode plaques at the interface of the electrolyte and electrode surface, and can be reduced by improving the electrode reaction efficiency which in turn enhances the diffusion rate of charged ions within the electrode.
If the capacity of the cell is governed by the amount of active material in the positive electrode, the cell is said to be of a positive electrode-limited type. Cells which are designed to consume the negative active material first are called negative electrode-limited. Typical nickel-metal hydride cells, for instance, are positive electrode-limited to reduce the chance of overpressurization if the cell is overcharged. As the cell is charged, oxygen is generated on the surface of the nickel hydroxide positive electrode and subsequently reduced by the metal hydride negative electrode. If the positive electrode is not charged up before the negative electrode, hydrogen gas can form at the negative electrode, resulting in high internal pressure. A typical ratio of negative-to-positive capacity is more than 1.6. In other words, a 650 milliampere-hour cell will typically contain enough negative active material (e.g., metal hydride alloy) to store 1040 milliampere-hours of energy. Some of this excess negative capacity is lost due to corrosion of the metal hydride alloy in the cell environment over the life of the cell.